PPI Calculator

What this tool does

PPI Calculator finds pixel density from a screen's diagonal size and horizontal/vertical resolution using the Pythagorean diagonal-in-pixels divided by the diagonal-in-inches, and also runs the calculation in reverse: enter PPI and resolution to get the diagonal size, or PPI, diagonal size and an aspect ratio to get the resolution. A reference table lists typical PPI ranges for common device categories.

Pixels per inch (PPI) measures how densely a screen packs its pixels, which affects how sharp text and images look up close. Specification sheets usually list it directly, but when they only give a diagonal size in inches and a horizontal-by-vertical resolution, working out PPI by hand means applying the Pythagorean theorem to the diagonal in pixels.

This calculator handles that forward calculation, plus two reverse lookups that come up when planning a display or image: given a target PPI and resolution, what diagonal size do you need, and given a target PPI, diagonal size and aspect ratio, what resolution should you aim for.

How to use PPI Calculator

1. For a forward calculation, enter the diagonal size, picking inches or centimetres in the side panel, and the horizontal and vertical pixel counts.
2. Read the pixel density and the diagonal measured in pixels under Find PPI from diagonal size & resolution.
3. To work out a diagonal size instead, enter a target PPI and a resolution under Find diagonal size from PPI & resolution.
4. To work out a resolution, enter a target PPI, a diagonal size and an aspect ratio such as 16:9 under Find resolution from PPI, diagonal size & aspect ratio.
5. Switch the unit in the side panel at any time; every diagonal field updates to match.
6. Scroll to the reference table for typical PPI ranges across monitors, laptops, tablets and phones.

Tips for better results

• The diagonal-in-pixels is always sqrt(horizontal² + vertical²), the same Pythagorean relationship used for the physical diagonal in inches or centimetres.
• Two screens with the same resolution but different physical sizes have different PPI; a 1920×1080 image looks sharper on a small laptop than on a large TV.
• When finding resolution from PPI and diagonal, the aspect ratio determines how the total pixel count splits between width and height, so get that ratio right first.
• PPI and the print term DPI (dots per inch) are often used interchangeably for screens, though DPI technically refers to printer output.

Common use cases

• Comparing the real sharpness of two monitors or phones that list different diagonal sizes and resolutions.
• Working out what diagonal size a panel needs to be to hit a target pixel density for a kiosk or signage project.
• Estimating the resolution needed for a new display given a desired PPI and known aspect ratio.
• Checking whether an exported image has enough resolution to look sharp at a given print or screen size.

Common mistakes to avoid

• Confusing a device's advertised resolution with its native panel resolution; some displays render at a lower internal resolution and scale up.
• Forgetting that viewing distance matters as much as PPI: a lower-PPI TV viewed from across a room can look just as sharp as a high-PPI phone held close.
• Using the wrong aspect ratio when solving for resolution, which skews the width and height even though the total pixel count is correct.